Terminology
Underbalanced drilling (UBD) is drilling a well with a drilling fluid hydrostatic head below the reservoir pore pressure. Managed pressure drilling (MPD) involved “low-head” and “at balance” drilling, in which the bottom hole pressure was kept marginally above or equal to the reservoir pore pressure. Reverse Circulation Center Discharge (RCCD) is drilling a well underbalanced while minimizing drilling fluid contact with the formation walls. Because all drilling can be considered managed pressure drilling in a generic sense, as used herein, Programmable Pressure Drilling (PPD), shall mean an adaptive well construction process used to precisely control the down hole annular pressure within specified environment limits by dynamically calculating, adjusting and applying a positive or negative pressure offset during drilling and while cementing. Further, a Programmable Gradient Drilling (PGD) system shall mean an adaptive well construction process that employs PPD methodology to thereby allow a variable pressure offset to be applied in a modulating fashion over incremental sections of well bore while drilling without disturbing the pressure within rest of the wellbore, resulting in a fully programmable annular pressure profile or gradient in which to further case and complete the well. PPD and PGD could further be understood as Programmable Automated Pressure Drilling (PAPD) or Programmable Automated Gradient Drilling (PAGD) with increasing utilization of automated process control loops.
PPD describes maintenance of bottom hole pressure at a specific pressure differential in the drilling zone. This is accomplished by utilization of a control unit and seal assembly. A stationary seal unit installed at a desired location in the borehole maintains pressure on the proximal side of both the control and seal unit at a pressure sufficient to control the well, provide sufficient flow to effect cooling of the bit using drilling mud being circulated through the drilling zone, and provide a flow rate sufficient to carry drill cuttings from the distal side of the control and seal units to the proximal side of the control and seal units and onwards back to the surface. The seal permits drilling to continue on the distal side of a movable control unit, while maintaining a pressure differential in the drilling zone from that pressure experienced on the proximal side of the seal.
PGD describes the additional aspect of an incremental deployment of a formation preserving seal on the formation wall, either chemical or mechanical in nature while performing PPD, to act as a pressure barrier as well as to strengthen the formation incrementally thereby simultaneously providing for a movable seal unit in close proximity to the movable control unit, where movement of both units is closely coordinated with the movement of the drilling assembly.
Conventional drilling practices or OBD have typically maintained the hydrostatic pressure of the drilling fluid in the well bore between the formation's pore pressure and its fracture pressure. Drilling fluid is continuously circulated within the well bore to control the formation fluids and transport cuttings to the surface. The drilling fluid also acts to stabilize the well bore and lubricates and cools the drill bit.
The present invention seeks to combine OBD to minimize the safety risks typically associated with UBD, such as H2S release, unforeseen and unplanned release of substantial quantities of hydrocarbons into the well bore (“a kick”) or where environmental regulations prohibit flaring or production while drilling, with the benefits of MPD or UBD in the drilling zone. Such methods avoid damage to the formation, lost circulation and all the other well-known problems. Moreover, the present invention avoids the need of drilling structure outfitted with the extra equipment commonly found with UDB or MPD programs, such as nitrogen injection units, closed tank batteries, multiphase separators, rotating choke devices, vacuum degassers and the like.
Typically drilling fluid is either a water-based or oil-based liquid and contains a variety of solid and liquid admixtures to impart density, fluid loss characteristics and Theological properties specific to the well bore conditions experienced or predicted. These conventional drilling methods have long been recognized as the safest way to drill a well despite recognizable problems created by this hydrostatic head of drilling fluid above a formation of interest. Since the drilling fluid pressure is higher than the natural formation pressure, fluid invasion frequently occurs causing permeability damage to the formation, caused by washout of the formation or physical blockage from the intrusion of the fluid and solids into the formation structure itself.
UBD was developed as drilling with a well bore fluid gradient less than the natural formation pressure gradient, which thereby permits the well to flow while drilling proceeds. This technique minimizes lost circulation and increases penetration rates while minimizing damage caused by the invasion of drilling fluid into the formation structure. Production zones are identified immediately and detailed well profiles can be formed from the progress of the drilling program in these underbalanced wells, leading to shortened drilling times—especially in marginal or older geological formations.
Reduced drilling time, increased bit life, early detection of formation changes and dynamic testing of productive intervals in the formation being drilled are enhanced by using UBD. Increased drilling efficiency, along with enhanced recovery prospects from undamaged formations, makes underbalanced drilling highly desirable.
UBD, as currently practiced, requires special surface equipment to safely and effectively drill. Density control of the drilling fluid is typically achieved by nitrogen injection into either the drill pipe or a parasite pipe. This requires significant surface preparation to effect appropriate nitrogen injection. Surface chokes to control bottom hole pressure can be employed to raise or lower the standpipe pressure, but the operation of the choke is not experienced by the bottom hole assembly because of the inherent lag time. The estimation of the lag time is normally straight forward for single phase systems, but multiphase flow systems are complex and difficult to model and hence difficult to predict their response, let alone control and manage it precisely.
UBD can result in a higher risk of blowout, fire or explosion if not managed properly; moreover, it requires rig crews to be fully trained in a totally different system, occupies large deck space and needs additional bed space which are normally very constrained offshore and is typically more expensive because of the additional surface equipment required for nitrogen injection and multiphase flow chokes and separation equipment. Yet, despite all of these problems, UBD is still widely used in modern drilling programs because the benefits far outweigh the costs.
MPD is known in the industry as a group of technologies to precisely control the annular pressure profile in the wellbore. The need to have precise control on the profile of annular pressure at all times during drilling and cementing is well established as it allows drilling through and completing complex pore and fracture pressure regimes, improving drilling efficiency due to reduced drilling risk and also avoids multiple expensive casing strings of reduced diameters to be installed in the wellbore. Earth formations undergo geological changes which result in unexpected pressure and rock strength variations over millions of years. In order to reach complex, deepwater and unconventional reservoirs, the industry needs new ways to drill through multiple different pore pressure and fracture gradients in the same hole section. Today, no technology exists that can change the annular pressure and keep it within desired limits at multiple fixed points in the wellbore while continuously drilling in the wellbore. The industry is aware of a constant bottom hole pressure system which maintains a desired pressure equal to or above the mud hydrostatic pressure at one point near the bottom of wellbore by applying a positive back pressure from surface on the annulus side to compensate for equivalent circulating density (ECD) reduction when surface pumps are stopped. Such method and associated apparatus do not allow dynamic reduction in bottom hole pressure as any reduction requires altering the mud hydrostatic which is a slow process. They also do not prevent these changes from impacting the annular pressure profile in the rest of the wellbore and its consequent adverse affects on the well bore integrity or inviting formation influx. The industry is aware of a dual gradient system which establishes a fixed point between surface and bottom hole where a change of gradient can be achieved by either injecting N2 using a parasitic string or a downhole pump. Not only is the capability of a dual gradient technique limited to just two gradients, the accuracy and precision in ensuring these gradients do not change during the course of drilling and completion is questionable due to many uncontrollable factors such as long open hole sections with compressible drilling fluids, lack of control on formation fluid influx, requirements to circulate continuously at all times and lack of downhole measurements all along the wellbore.
The present invention seeks to obtain all of the benefits of underbalanced drilling accompanied by all of the safeguards of conventional overbalanced drilling by controlling pressure adjacent the drill bit and bottom hole assembly and sealing and/or strengthening the formation while drilling. The present invention also seeks to overcome the shortcomings in current MPD practices by precisely controlling the annular pressure profile throughout the well bore. The invention also provides a unique solution to the industry for all the problems and costs associated with design, cleaning and maintaining drilling fluids. The present invention also provides the industry a solution to safely drill exploration wells underbalanced thereby increasing the chance of finding new productive zones previously overlooked by conventional OBD techniques.
The drilling industry has long sought a solution to these problems. For example, U.S. Pat. No. 5,873,420 discloses using a control valve adjacent the drill bit to release air into the mud mixture to lighten the hydrostatic head based upon sensed bottom hole pressure and other fluid measures. When open bottom hole pressure reached unsafe levels, the air supply would be reduced or eliminated thereby relying upon the column of dense drilling mud to control the formation pressure.
Similarly, U.S. Pat. No. 6,732,804 discloses a dynamic mudcap system utilizing concentric casings which allow a column of drilling mud to be maintained in a well bore annulus to control the well from blow out. The patent also discloses using a deployment valve to seal off bottom hole portions when the drill bit assembly is pulled for service or replacement. In neither prior art device is there any disclosure of means for open hole formation strengthening or preservation.
Prior art down hole plug arrangements intended to protect open hole underbalanced drilling, such as shown in U.S. Pat. Nos. 5,954,137 and 7,086,481, have required drill string manipulation to set and release the downhole plug.